This invention relates to Fe-based, soft magnetic alloys and a dust core of said alloy.
Conventionally, iron cores of crystalline materials such as permalloy or ferrite have been employed in high frequency devices such as switching regulators. However, the resistivity of permalloy is low, so it is subject to large core loss at high frequency. Also, although the core loss of ferrite at high frequencies is small, the magnetic flux density is also small, at best 5,000 G. Consequently, in use at high operating magnetic flux densities, ferrite becomes close to saturation and as a result the core loss is increased.
Recently, it has become desirable to reduce the size of transformers that are used at high frequency, such as the power transformers employed in switching regulators, smoothing choke coils, and common mode choke coils. However, when the size is reduced, the operating magnetic flux density must be increased, so the increase in core loss of the ferrite becomes a serious practical problem.
For this reason, amorphous magnetic alloys, i.e., alloys without a crystal structure, have recently attracted attention and have to some extent been used because they have excellent soft magnetic properties such as high permeability and low coercive force. Such amorphous magnetic alloys are typically base alloys of Fe, Co, Ni, etc., and contain metalloids as elements promoting the amorphous state, (P, C, B, Si, Al, and Ge, etc.).
However, not all of these amorphous magnetic alloys have low core loss in the high frequency region. Iron-based amorphous alloys are cheap and have extremely small core loss, about one quarter that of silicon steel, in the frequency region of 50 to 60 Hz. However, they are extremely unsuitable for use in the high frequency region for such applications as in switching regulators, because they have an extremely large core loss in the high frequency region of 10 to 50 kHz. In order to overcome this disadvantage, attempts have been made to lower the magnetostriction, lower the core loss, and increase the permeability by replacing some of the Fe with non-magnetic metals such as Nb, Mo, or Cr. However, the deterioration of magnetic properties due to hardening, shrinkage, etc., of resin, for example, on resin molding, is large compared to Co-based alloys, so satisfactory performance of such materials is not obtained when used in the high frequency region.
Co-based, amorphous alloys also have been used in magnetic components for electronic devices such as saturable reactors, since they have low core loss and high squareness ratio in the high frequency region. However, the cost of Co-based alloys is comparatively high making such materials uneconomical.
As explained above, although Fe-based amorphous alloys constitute cheap soft magnetic materials and have comparatively large magnetostriction, they suffer from various problems when used in the high frequency region and are inferior to Co-based amorphous alloys in respect of both core loss and permeability. On the other hand, although Co-based amorphous alloys have excellent magnetic properties, they are not industrially practical due to the high cost of such materials.
In the technical field of dust cores, use is made of iron powder, Mo permalloy, etc. for dust cores in noise filters and choke coils, since they can be produced in a variety of shapes more easily than can thin strips. However, there are problems in their use in power sources at high frequency owing to the comparatively large core loss.
As described above, Fe-based amophous alloys constitute an inexpensive soft magnetic material, but their magnetostriction is comparatively large, and they are inferior to Co-based amorphous alloys in respect of core loss and permeability, so that there are problems in using these materials in the high frequency region. On the other hand, although Co-based amorphous alloys have excellent magnetic properties, as hereinbefore pointed out, the high price of the raw material makes them commercially disadvantageous. Also, such materials also suffer disadvantages where used for dust cores since they too have comparatively large core losses, causing problems in their use in power sources of high frequency.